1. Field of the Invention
The present invention relates to a heterocyclic compound. The present invention also relates to a light-emitting element, a light-emitting device, an electronic device, and a lighting device each using the heterocyclic compound.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In the basic structure of such a light-emitting element, a layer which contains a light-emitting substance is interposed between a pair of electrodes. By voltage application to this element, light emission from the light-emitting substance can be obtained.
Since such light-emitting elements are self-luminous elements, they have advantages over liquid crystal displays in having high pixel visibility and eliminating the need for a backlight, for example, thereby being considered as suitable for flat panel display elements. Light-emitting elements are also highly advantageous in that they can be thin and lightweight. Furthermore, very high speed response is one of the features of such elements.
Furthermore, since such light-emitting elements can be formed in a film form, they make it possible to provide planar light emission. An element having a large area can thus be formed. This is difficult to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, light-emitting elements have great potential as planar light sources applicable to lightings and the like.
Light-emitting elements using electroluminescence can be broadly classified according to whether they use an organic compound or an inorganic compound as a light-emitting substance. In the case where an organic compound is used as the light-emitting substance, application of a voltage to a light-emitting element causes injection of electrons and holes into a layer that includes the organic compound having a light-emitting property from a pair of electrodes, and thus a current flows. Then, carriers (electrons and holes) recombine, thereby forming the excited state of the organic compound having a light-emitting property. When the excited state is changed to the ground state, light is emitted.
The excited state generated by an organic compound can be a singlet excited state or a triplet excited state. Luminescence from the singlet excited state (S*) is referred to as fluorescence, and luminescence from the triplet excited state (T*) is referred to as phosphorescence. In addition, the statistical generation ratio thereof in a light-emitting element is considered to be as follows: S*:T*=1:3.
With a compound that can convert energy of a singlet excited state into light emission (hereinafter, called a fluorescent compound), only light emission from the singlet excited state (fluorescence) is observed and that from the triplet excited state (phosphorescence) is not observed, at room temperature. Therefore the internal quantum efficiency (the ratio of generated photons to injected carriers) of a light-emitting element using a fluorescent compound is assumed to have a theoretical limit of 25% based on the ratio, S*:T*=1:3.
In contrast, with a compound that can convert energy of a triplet excited state into light emission (hereinafter, called a phosphorescent compound), light emission from the triplet excited state (phosphorescence) is observed. Further, with a phosphorescent compound, since intersystem crossing (i.e., transition from a singlet excited state to a triplet excited state) easily occurs, the internal quantum efficiency can be increased to 75% to 100% in theory. In other words, the emission efficiency can be 3 to 4 times as much as that of an element using a fluorescent compound. For this reason, light-emitting elements using phosphorescence compounds are now under active development in order to realize light-emitting elements having higher efficiency (e.g., see Patent Document 1).
When a light-emitting layer of a light-emitting element is formed using a phosphorescent compound described above, in order to suppress concentration quenching or quenching due to triplet-triplet annihilation in the phosphorescent compound, the light-emitting layer is often formed such that the phosphorescent compound is dispersed in a matrix of another compound. Here, the compound serving as the matrix is called a host material, and the compound dispersed in the matrix, such as a phosphorescent compound, is called a guest material.
In the case where a phosphorescent compound is a guest material, a host material needs to have higher triplet excitation energy (energy difference between the ground state and the triplet excited state) than the phosphorescent compound.
Furthermore, since the singlet excitation energy (energy difference between the ground state and the singlet excited state) is higher than the triplet excitation energy, a material that has high triplet excitation energy also has high singlet excitation energy. Therefore the above substance that has high triplet excitation energy is also effective in a light-emitting element using a fluorescent compound as a light-emitting substance.
In addition, a light-emitting element having high current efficiency is expected to realize a light-emitting device, an electronic device, and a lighting device each having low power consumption.